Characterization of Hydrogen in Basaltic Materials With Laser-Induced Breakdown Spectroscopy (LIBS) for Application to MSL ChemCam Data

The Mars Science Laboratory rover, Curiosity, is equipped with ChemCam, a laser-induced breakdown spectroscopy (LIBS) instrument, to determine the elemental composition of nearby targets quickly and remotely. We use a laboratory sample set including prepared mixtures of basalt with systematic variation in hydrated mineral content and compositionally well-characterized, altered basaltic volcanic rocks to measure hydrogen by characterizing the H-alpha emission line in LIBS spectra under Martian environmental conditions. The H contents of all samples were independently measured using thermogravimetric analysis. We found that H peak area increases with weight percent H for our laboratory mixtures with basaltic matrices. The increase is linear with weight percent H in the mixtures with structurally bound H up to about 1.25wt.% H and then steepens for higher H-content samples, a nonlinear trend not previously reported but potentially important for characterizing high water content materials. To compensate for instrument, environmental, and target matrix-related effects on quantification of H content from the LIBS signal, we examined multiple normalization methods. The best performing methods utilize O 778- and C 248-nm emission lines. The methods return comparable results when applied to ChemCam data of H-bearing materials on Mars. The calibration and normalization methods tested here will aid in investigations of H by LIBS on Mars with ChemCam and SuperCam. Further laboratory work will aid quantification across different physical matrices and heterogeneous textures because of differences we observed in H in pelletized and natural rock samples of the same composition. Plain Language Summary ChemCam, one of the Curiosity rover instruments, measures the chemical composition of nearby targets quickly and remotely using laser-induced breakdown spectroscopy (LIBS). The LIBS technique requires laboratory calibration to quantitatively determine the abundances of major and minor elements. We measured prepared mixtures of basalt with systematic variation in hydrated mineral content and compositionally well-characterized, altered basaltic volcanic rocks with a ChemCam-analog instrument under Martian environmental conditions to characterize LIBS hydrogen emission. To compensate for instrument, environmental, and target-related effects on quantification of H content from the LIBS signal, we tested multiple normalization methods. While the H signal increased nonlinearly with weight percent for high H samples, we found laboratory calibrations that allowed us to successfully determine the H content of Martian veins and bedrock measured by ChemCam.